Novel pneumatic cylinder design could halve air losses

Researchers at the University of Liverpool in the UK believe they have found a way to halve the air used by pneumatic cylinders, while making them respond faster.

The researchers, led by Dr Jihong Wang of the University`s Department of Electrical Engineering and Electronics, set out to reduce the heavy energy losses incurred in conventional pneumatic cylinders. Less than 30% of the energy required to produce the compressed air does useful work at the actuator end. The rest is wasted.

The air released to the atmosphere by the cylinder exhaust is still at a relatively high pressure, but there is no practical way to harness it.

Although the movement of a pneumatic cylinder is linear, pneumatic actuators are actually non-linear systems, making them difficult to control. Dr Wang and a PhD student, Jai Ke, used software developed in-house to model how actuators work. They used this to devise new control strategies, but the efficiency improvements were limited to a maximum of 1.5%.

Although this could generate appreciable savings if applied to millions of actuators, Dr Wang felt that she could do better.

In traditional cylinder designs, the acting force can be strengthened by increasing the driving chamber pressure and the piston area. However, when the piston area is increased, the chamber volumes also increase, needing more compressed air to achieve the required pressures. "This means that in traditional linear actuators, increasing the acting force results in more compressed air being consumed," Wang explains.

To overcome this, she designed a series of rodless pneumatic cylinders consisting of a cylinder, guided for reciprocal movement along one axis, a tubular element forming a chamber on one side of the piston, and a port to transfer air to and from the chamber. Inside the cylinder (shown schematically above), there is a stretchable, corrugated material, whose pleats close up as the piston approaches either end of the cylinder, thus acting as a shock absorber.

"The chamber volumes are small compared to those of traditional actuators with the same piston area," Wang says. "This means less compressed air is needed to build up the required chamber pressure  yet it can provide the same acting force."

Dr Wang and her students have carried out a series of simulation studies to compare the efficiency of the new design with conventional cylinders operating under similar conditions.

"Our results consistently indicate that the new pneumatic cylinder design would consume up to 50% less compressed air when doing the same job as a traditional pneumatic cylinder," she reports. The graph above shows the compressed air consumption of a typical conventional pneumatic cylinder, compared to the new design.

"We also found that the new design has faster responses," Dr Wang adds, "because it requires less time to charge the smaller chamber volume  offering higher timing efficiency than the traditional design."

She has also designed a rodded version of her actuator. The University of Liverpool has applied for patents to cover various aspects of the designs.

The next step is to build prototypes and have them tested by potential users. The University is talking to manufacturers interested in adding the novel cylinders to their ranges. It would also like to hear from other potential manufacturers and users. If you are interested in exploiting the technology, contact Dr Gillian Murray at Gillian.Murray@liverpool.ac.uk